In positive displacement compressors, discrete volumes of gas are trapped and compressed with the trapped, compressed volumes being discharged from the compressor. The trapping of the volumes at suction pressure and their discharge at discharge pressure each produce pressure pulsations and the related noise generation. In the case of chillers, the suction pipe extends into the cooler and the suction gas pulsation in the cooler has been found to be one of the major noise sources in a chiller. This noise source becomes significant after vibration and acoustic treatments have been performed to control compressor vibration and discharge gas pulsation utilizing compressor isolators and a discharge muffler.
The flow of gas is along a flow path defined by a pressure differential and, for the suction flow, is through the suction pipe into the compressor. The direction of noise generation is not dictated by the flow direction.
The gas pulsation resulting from the intermittent nature of gas intake is exacerbated by variable speed operation which greatly extends the frequency range over which noise can be generated during operation. A suction inlet muffler, an acoustic treatment or lagging the cooler, partially or completely, can be employed for noise attenuation. While an absorptive suction muffler is an obvious choice, they are made to attenuate noise in a particular frequency range, or ranges, much less extensive than the frequency range associated with variable speed operation.
In the suction pipe of a chiller compressor the gas flow and the gas intake noise pulsations are traveling in primarily opposite directions, although some acoustic energy is reflected back towards the compressor where the suction pipe terminates in the cooler. The present invention locates a dissipative-type muffler at the inlet end of the suction pipe which is within the cooler. The inlet cross section of the muffler is oversized so as to permit a series of reductions in cross section down to the cross section of the suction pipe which is suitable for feeding the suction inlet of the compressor. The changes in the areas of the cross sections are primarily for reducing flow losses but could have slight acoustic benefits as where the wave propagation in the suction pipe is highly modal in nature, i.e. only is beneficial where the pipe cross section is small compared to an acoustic wavelength of 300 Hz, or less. The flow path through the muffler into the suction pipe involves a number of changes in flow direction. At each directional change in the muffler, the cross section of the flow path is decreased in the direction of flow. A preferred area reduction at each directional change is on the order of one third which keeps the flow and turning losses small.
It is an object of this invention to provide enhanced muffler performance.
It is another object of this invention to attenuate suction gas pulsation in a variable speed chiller compressor. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, the gas flow direction and the noise generation direction are in opposite directions in the suction muffler. The flow path in the muffler has a number of changes in direction and the flow path cross section decreases at each change of direction in the direction of flow.